The invention concerns a support material for black-and-white or color photographic emulsions, the support material being of the type that includes at least one coating hardened by means of electron radiation.
A number of support materials for photographic emulsions are known. Among the most common supports are film materials and papers. Papers usually have extra coatings. Watertight papers coated with layers of synthetic resin in particular are widely used as supports for photographic emulsions. Synthetic films and laminates with special coatings are also known. The most widely used have been papers that are coated on both sides with polyolefin resins and film materials that are coated with pigmented layers, such being described, for example, in DE-AS No. 14 47 815, U.S. Pat. No. 3,833,380, U.S. Pat. No. 3,630,742 and U.S. Pat. No. 3,928,037. However, such coatings have only a limited pigment capacity, are not scratch-proof and are not suitable for thermal image development. Considerably improved paper supports consisting of base paper with at least one coating produced in situ by means of electron radiation hardening are described in DE-OS No. 30 22 451 and DE-OS No. 30 22 709. The advantages of a coating hardened with electron radiation are its resistance to scratching, its high pigment acceptance capacity and its enhanced surface sheen in comparison with polyolefin coatings.
Synthetic resin coatings are resistant not only to water, but also to acid and alkaline photographic processing solutions to thereby prevent the penetration of these solutions to paper supports underneath them. Consequently, in the case of paper or unwoven material bases, the time-consuming washing operation is decisively shortened.
All resin coatings, whether on paper or on film, can contain pigments, dyes, optical brighteners, image stabilizers, antioxidants, or other additives, to the extent that each may be desirable or necessary with respect to the desired characteristics of the image to be formed on the support material. Of the possible additives, pigments and dyes are the most important for the visual impression of a photographic image on the coating. In the case of color images, they determine the color character and are decisive for the sharpness of the photographic images.
The photographic emulsions, after appropriate pretreatment of the resin surface, are applied directly to the surface of the resin layer or they are applied after the application of an adhesion-promoting intermediate coating to the surface of the resin layer. These photographic emulsions are preferably those known under the concept of silver salt photography and can be used to produce black-and-white or color images.
Resin coatings hardened with electron radiation according to DE-OS No. 30 22 451 or DE-OS No. 30 22 709 are produced by the uniform application of a flowable mixture to the surface of a paper or other base support and subsequent hardening with high-energy electron radiation under a buffer gas or some other shielding agent. The mixtures contain as the decisive constituent at least one substance with ethylenically unsaturated double bonds which have the capability to undergo a polymerizing reaction when it is triggered by radiation.
Although the electron radiation-hardened coatings have many advantages compared with polyolefin coatings, they still are limited by many drawbacks. Despite the great variety in the composition of radiation-hardened coatings, it has not been possible to produce a coating that behaves uniformly well in all photographic processing solutions. Rather, it has been shown that coatings hardened with electron radiation behave differently with different processing solutions. There are commercially available color developing preparations which, under the subsequent influence of oxygen, lead to a yellowish discoloration of the surface of the radiation-hardened coating. This discoloration is admittedly slight, but it is clearly visible and cannot be prevented by either the stop bath or thorough washing. Such discoloration occurs mainly with the use of photographic developing solutions containing an aromatic amine derivate, such as derivates of phenylene diamine, toluidine, and the like. Consequently, the use of papers with radiation-hardened coatings has been limited heretofore to processes in which other chemicals are used for image development, such as, for example, hydroquinone.
This yellowing by color developers does not occur in the case of polyolefin surfaces that contain rutile or anatase or another white pigment, nor does it occur in conventional baryta papers having at least one coating consisting essentially of barium sulfate and gelatine. Other coatings consisting of different synthetic resins and white pigment also do not show this coloration, while heretofore described radiation-hardened coating based on acrylates, methacrylates or allyl compounds are always visibly discolored with color developers. Accordingly, the composition of the binder component seems to be decisive for the discoloration with developers, especially a binder component having the possible presence of unsaturated organic compounds.
The discoloration of radiation-hardened coatings by color developers shows up in both pigment-free coatings and coatings that contain white pigment. As a rule, the discoloration in pigment-containing coatings is even greater than in pigment-free coatings, as shown in Table 1 below, which reports discoloration data for binders that included 62% by weight of polyester tetraacrylate, 22.5% by weight of glycerin propoxytriacrylate, and 15.5% by weight of hexandioldiacrylate.
TABLE 1 ______________________________________ Coating discoloration by color developers Discoloration ______________________________________ Layer without pigment 0.05 Layer with 20% by weight BaSO.sub.4 0.10 Layer with 20% by weight ZnS 0.11 Layer with 20% by weight CaCO.sub.3, type 1 0.09 Layer with 20% by weight CaCO.sub.3, type 2 0.075 Layer with 20% by weight TiO.sub.2 (anatase) 0.08 Layer with 20% by weight TiO.sub.2 (rutile 1) 0.06 Layer with 20% by weight TiO.sub.2 (rutile 2) 0.07 Layer with 20% by weight Zn--titanate 0.65 Layer with 20% by weight Al.sub.2 O.sub.3 0.075 Layer with 20% by weight AlO (OH) 0.08 Layer with 20% by weight SiO.sub.2 0.065 Layer with 20% by weight MgO 0.06 Layer with 20% by weight ZnO 0.085 ______________________________________
Copending U.S. patent application Ser. No. 722,732, filed Apr. 11, 1985, revealed a way to obtain coatings with diminished discoloring by using special compositions of the hardenable components of a coating mixture. The procedure described therein is based on the fact that unsaturated hydroxyfunctional compounds are contained in a molar concentration of 2 or greater in the coating mixtures. The technical principle of this approach is basically applicable to both pigment-free and pigmented coating mixtures. Certainly, pigment-containing coating mixtures as a rule are discolored somewhat more than pigment-free coatings. Therefore, the OH-concentration in pigmented coating should be slightly greater than in comparable pigment-free coatings.
The disadvantage of the solution provided by this copending application, however, is the greatly limited choice of usable mixture components. This disadvantage becomes especially apparent in the case of highly pigmented coatings which, in order to avoid undesirable brittleness with increasing pigment content, require an increasing quantity of flexibilizing additives.
It is therefore an object of the present invention to provide a radiation-hardened coating layer which provides an exceptionally versatile application of the coated support which, even after usual treatment with commercially available color developers, shows little or no discoloration.
This object is achieved by using as white pigment a titanium dioxide that has an inorganic surface coating which constitutes at least 2% by weight of total pigment.
Titanium dioxide pigments used in coatings can be untreated like the TiO.sub.2 (rutile 2) listed in Table 1. Frequently, however, for improvement of their dispersability, they are given a surface aftertreatment; that is, they are coated with inorganic oxides such as Al.sub.2 O.sub.3, AlO(OH), SiO.sub.2, ZnO and others. Here, hydrated oxides are also to be understood to be included under the term oxides in its broader sense. Such surface-coated types are, for example, the TiO.sub.2 (anatase) and TiO.sub.2 (rutile 1) listed in Table 1 which are coated with 0.6% and 1.0% by weight, respectively, of aluminum oxide and, like the other white pigments of Table 1, lead to increased discoloration with color developer. Even the use of titanium dioxides that are thus treated or that also are surface coated with organic substances does not lead to a decrease in the discoloration with a color developer unless OH-containing compounds as indicated in the copending application are present at the same time.